Luminous discharge tube



Sept. 11, 1951 J. w. MITCHELL 2,567,491

' LUMINOUS DISCHARGE TUBE I Filed Sept. 6, 1945 3 Sheets-Sheet 1 Inventor Tum WssuaY Nncnsu,

A llorneys Sept. 11,1951 J. w. MlTC HELL 2,567,491

LUMINOUS DISCHARGE TUBE Filed Sept. 6} 1945 SSheec's-Sheet 2 j Inventor Tom: WELEY M ITCHELI L A llorneys P 11, 1951 J. w. MITCHELL 2,567,491

I LUMINOUS DISCHARGE TUBE Filed Sept. 6, 1945 s Sheets-Sheet 5 Inventor 38H M \A/EsLEY M lTdHEbb awaw A llorneys Patented Sept. 11, 1951 UNITED STATES PATENT OFFICE Application September 6, 1945, Serial No. 614,720 In Great Britain December 29, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires December 29, 1963 12 Claims. (Cl. 313-184) This invention relates to luminous electron discharge tubes of the cold-cathode type designed to produce luminous flashes of high intensity and extremely short duration.

Discharge tubes of the character in question are mainly intended for use as the light source for high speed photography by reflected light, particularly in connection with the study of ballistic phenomena, and they usually have a pair of main electrodes enclosed in an evacuated envelope with a filling of an inert gas. The discharge can be initiated either by deriving from a suitable external circuit a very high voltage pulse which is applied across the main electrodes, or by applying a steady high voltage across these electrodes suflicient to produce the discharge when a triggering voltage is applied to an auxiliary triggering electrode situated inside or outside the tube. Hitherto considerable difliculty has been experienced in obtaining reliable starting with such tubes. A tube may fail to discharge the condenser supplying the high voltage or may break down spontaneously or, if it does not fail in these respects, the time delay between the occurrence of the event to be photographed and the occurrence of the actual flash may be too variable for practical purposes. By using an external starting electrode in the form of a band surrounding the tube, the starting characteristics can be improved provided that the pressure or the inert gas is not too high. At high gas pressures another difliculty arises in that the discharge tends to occupy a broad jagged channel passing along the wall of the tube from the starting band to the anode, causing sintering and crazy cracking of the envelope and thus limiting the life of the tube. With lower gas pressures however, the duration of the main discharge is too long to allow photographs of bullets in flight to be taken if a sufficiently large condenser is discharged to provide enough light for this purpose. In this connection it must be pointed out that to obtain satisfactory photographs of a bullet travelling at a velocity of about 3000 to 3500 feet per second the duration of the main flash should not appreciably exceed one microsecond, and the intensity of any afterflow relatively to that of the main flash must be sumciently low to ensure that under normal circumstances it does not give rise to an image on the photographic film.

An object of the invention is to provide luminous electron discharge tubes which are substantially free from the defects mentioned above; that is to say, tubes possessing absolutely reli- 2 able starting, freedom from sintering and sputtering of the envelope, a main flash of duration and intensity suitable for photographing projectiles in flight, and a reasonably long useful life.

Another object of the invention is to increase the useful life of a discharge tube of the character referred to by subjecting the electrodes, thereof to a preliminary deep etching treatment, whereby, upon subsequent activation of the electrodes, the metallic constituent of the activating material penetrates deeply into the structure of the electrodes and so provides a reserve for replenishing the surface layer of activating material.

Basically, the invention comprises a coldcathode flashing electron discharge tube having at least two activated electrodes mounted within an enevelope containing an inert gaseous filling at a pressure lying within the range of from about 0.1 atmosphere to about 3 atmospheres, these eelctrodes being so disposed that their axes of symmetry coincide with the axis of symmetry of the tube and being composed wholly or in part of material which is either permeable to a metallic constituent of the activating agent or readily forms a stable alloy therewith. The activating agent takes the form of electron-emitting material of very low workfunction which is either not destroyed by the discharge or else is automatically renewed during the life of the tube. It is deposited in layer form on the electrodes and preferably consists of, or contains, an alkali metal, the electrodes being saturated with this metal but without excess of free alkali metal being present.

The electrodes are preferably made from copper, nickel, cupronickel alloys or silver, and it has been found that the most satisfactory results are obtained by using a potassium-potassium oxide layer on heavily oxidised copper, nickel or silver; a potassium or potassium amalgam layer on copper, nickel or silver; a potassium-potassium hydride layer on copper, nickel or silver; or layers similar to those mentioned in which potassium is replaced by another alkali metal.

It has been found that the use of activated electrodes not only renders the starting of the tube quite reliable, and removes triggering delays and uncertainties and permits the use of lower triggering voltages, but also increases the peak luminous intensity'of the discharge relatively to the intensity of the afterglow, thus increasing the effective light output of the tube; allows a discharge of far higher energy to be passed without damaging either the tube or the electrodes. and eliminates sputtering oi the electrode material.

In one iorm oi construction, in accordance with the invention. the tube has three activated electrodes one oi which is in the iorm oi a rim or hollow cylinder surrounding another oi the electrodes which is rod-shaped. Preferably. the end oi the rod-shaped electrode protrudes a short distance beyond the surrounding electrode towards the remaining electrode. Such an arrangement oi the activated electrodes results in thedischargebeingconfinedtoaverynarrow channelclosetotheaxisoithetube. Further. activated auxiliary electrodes each in the iorm oi a ring or hollow cylinder may be provided alongthedischargepathbetweentheanodeand cathode. These auxiliary electrodes are intended to have potentials lying between the cathode and anode potentials, the one neamt the anode being at the highest potential. This arrangementenablesadischargeoigreatlyincmd length to be obtained.

'Ihepressureoitheinertgaseousfillingshould beashlghaspoesiblewithintheraugeoipressures already specified, taking into account the mechanicalstrengthoithedischargetubeand the ease oi triggering. Argon is the preierred filling, but the filling could also be helium; neon; or mixtures oi the iollowing components: (a) helium and (b) neon: (a) neon and (in argon; (a) argon and (b) mercury vapour; (a) argon and (b) hydrogen; (a) argon and (b) kryptonxenon; or (a) argon and (b) krypton-xenon-hydrogen. Where mixtures such as stated above are used, the pressure oi component (b) oi each mixturewillliebetweenairactionoioneper cent and 25 per cent.

, The preierrediormsoiconstructionin accordance with the invention, and the'best methods oi carrying it into eiiect, will now be described in detail, and by way oi illustration, with reierence to the annexed drawings. in which:

Figures 1 and 1A show respectively longitudinal and transverse sections oi a three-electrode discharge tube;

Figure 2 is a circuit diagram oi an operating circuit ior a three-electrode discharge tube;

Figures 3 and 4 show a two-electrode discharge tube and alternative operating circuits ior use therewith;

Figureiishowsadischargetubewithauxiliary electrodes and an operating circuit ior use therewith; and

Figures 6 to 8 show longitudinal sections oi three iurther examples oi cold-cathode triode discharge tubes in accordance with the invention.

ReierringtoFlgures1and1A,thisi'ormoi dischargetubehasanenvelopeloiaheatraistantglassoianinternaldiameteroifll mm. Thelumlnousdischargeoccursbetween two main electrodes 1, l which are separated byadistanceoi35mm. Theelectrodelisin theiormoiarodcomposedoi lfigaugepure nickelwireconnectedtoaleadloitungsten,

a nickel sleeve being slipped over the joint and spot-welded. The tipoithe electrode Iis rounded andpclished. The electrode 3 is in the iorm oi an open-ended cylinder of copper dllmmwideanditmmininternaldiameter,

andissilver-solderedtoahrngstenleadt. The

thirdelectrodelisanopen-endedcoppercylinder ummwideanditmmininternaldiameter whichsurroundstheelectrodelthelatterpro- 4 tungstenleadlissilver-solderedtotheelectrode Landiiexiblestrandedcopperwireleablailversolderedtothetimgstenleada The various dimensions quoted above are not critical: ior example,theseparationbetwemthe mainelectmdesl, lcanbeiromaotoai'imm.

vior working potentials of 3500 to 7500 volts.

Greatcarehastobetakenduringtheaealinginoitheelectrodestoensmethattheyarevu-y accurately aligned with their am at symmetry coincidentwith the longitudinal axis oi the tube, jigs being employed ior thk m 'I'hefirststepinconstructingthediseharge tubeistoprepare thetungstenglass-metalseab. Thetungstenleadstiandlareeachaomm. long, andoi 18 gauge. Theyarecleanedhybeing repeatedly warmed and dipped in, a solution oi potassium nitrite, and then washed. dried and oxidised in a broad flame until the bright interference colours just disappear. A glass sleeve is then slipped on and fused to the wire, using a small amount of oxygen in the blowpipe M. 11' intermediate glasses are not available, the glam shouldbethesameasthatwhiehistobeused for the envelope of the tube. The colour oi the seal at this stage should be bright orange red. Abead oiglass isiusedonatadistanceoiflto'l mm. irom one end.

The envelope I is formed from heat-resistant glass tubing about 15 to 20 cm. long, from 2! to 30mm.inexternaldiameterandhavingawall thicknessoiirom 1.5 to2mm. Onaccountoithe need for high mechanical strength. the tubing should be of uniform wall thickness and tree ircm streaks, knots and air bubbles. After thoroughly cleaning the tubing, one end is rounded and its wall thickened to at least 2 mm. Then the side arm is sealed on, care being taken to produce a strong joint of uniform thickness. Next, after the electrodes have been subjected to the preliminary treatment specified below, the electrode lissealedin. Itisheldaccuratelyontheaxis oithetubebyametalroddrilledatoneendto taketheelectrodethisrodbeingitseliheldin positionbyasleeveandacorkintheopenend of the tube. The metal rod in question has an external diameter equal to the of the cylindrical electrodes 3 tion is held in position by an external Jig the metal rod. Then the top section of is annealed and the electrode 3 sealed in. After this, the open end of the tube is rounded and enclosed, and the complete structure is thoroughly annealed.

A description of the activation oi the electrodes will now be given. They are first etched with dilute nitric acid until the crystalline structure of the copper or nickel is clearly developed. They are then washed and dried-and the tube is pumpedtoahardvacuumandbakedinaiurnace at 350 C. for 30 minutes. Oxygen at a pressure of a few millimetres oi mercury is then introduced intothetubeandtheelectrodesoxidisedbymeans of a high-voltage glow discharge passed a between them. the gas pressure being adjusted so that the glow completely covers the cathode Iandthegrid 'Lwhichareconnectedbgcther during this operation. Thh treatment is continued until the electrodesassumeadarkbrown colour-,anditisessentialtoavoidanysputtering oithewalls. Thetubeisthenpumpedtoahard vacuumandpotassiumdistilledintoitbywayoi the side arm previously mentioned, the tube being trudingthrough it ioradistanceoilmm. A 5 keptat350C.ior10tol5minntes. Duringthh r s time the potassium redo the oxide. layers on the electrodes and penetrates-to a considerable depth along the inter-crystalline surfaces of the copp r and nickel. Allexcess free-potassium 13 then carefully removed by pumping while the tube is cooling. any metal condensing on the walls being distilled away by warming with a gas flame.

amp e 7 positive voltage derived from a photocell impulse unit can be applied to the grid.

As shown in Figure 2, the rod electrode 2 acts as the'cathode and the cylindrical electrode 3 as the anode. This is the preferred method of working, butthe connections can be'reverse'd so This is essentiaL. because if any potassium remains on the wall of the tube the discharge will pass along the-wall, which usually leads to destruction of the tubeafter a few flashes. Likewise, if excess potassium is left on the electrodes it is sputtered onto the wall of the tube by the first few discharges, with a similar result. After cooling to room temperature; the tube is filled with argon at atmospheric. or high pressure, tested for triggering and stability and sealed off. The

As a further example, the preparation of a tube having its electrodes activated with potassium hydride will be described. After the etching of the electrodesand the baking of the tube, as

above, pure potassium metal is distilled into the tube and baking-is then continued for 10 to 20 minutes, during which time the potassium becomes completely absorbed by the electrodes. The tube is now allowed to cool; hydro en at a pressure of approximateiy'2 mm. of mercury is introduced, and a glow discharge of low current density is passed to form layers of potassium hydride on the electrodes. The tube is finally filled at 0 C. with hydrogen at a pressure of 50 mm. of mercury, and pure argon at a pressure of 710 mm. of mercury, and sealed oil.

In the case of a discharge tube of the type shown in Figure 1, any circuit which is capable of producing a pulse of steeply rising leading edge and having a maximum amplitude exceeding 25 kilovolts, can be used for triggering the tube. An operating circuit of this character is shown in Figure 2.

A condenser ll of two microfarads is charged from the 7500 volt supply through a resistance l2. of two megohms, this condenser being connected across the electrodes 2, 3 of the tube I. The tube is triggered to discharge thi condenser and thus produce the flash by applying a triggering pulse of 3000 volts to the grid 1, this pulse being derived from the triggering circuit iii. In this circuit an 0.1 microi'arad condenser 1'3 is charged from the 250 volt supply through a 100,000 ohm resistance II and is discharged through the primary winding of a high-ratio induction coil I! by removing the negative bias from the grid of a gas-filled triode' l6 connected in series with a 100 ohm resistance H. The induction coil l5 may, if desired, be replaced by a 4:1 ratio pulse transformer which, when incorporated'in a circuit of the same type as that shown in Figure 2, can develop in its secondary winding a pulse having a steeply rising leading edge and a maximum amplitude of at least 3000 volts. Any suitable means for removing the negative bias from the valve It can be adopted, this depending largely on the event being photographed; For example, a make-break contact such as a rupture or ballistic screen can be arranged to short-circuit terminals It in series with a 1000 ohm resistance that the rod acts as thefanode. Inthis case the secondary winding .cf the induction coil l5 or transformer is isolated from earth and connected between anode and grid so as to apply a negative pulse to the grid, although the tube will trigger with either a positive or negativepulse.

Typicalperformance figures for the three-elec-r trode tube described with reference to Figures 1 and 1A operating at 7500 volts are as follows. The

photographically-eflective duration of the main flash is one microsecond and its intensity is such that an area of at least 500 square feet at a disstance of 20 'feet' from the camera and tube is sufficiently illuminated to allow satisfactory single photographs to be taken at an aperture of F8 or smaller, using standard high-speed photographicw emulsions. The delay between the event pulse and the flash does not exceed 6 microseconds, and this delay is constant to within :1 microsecond.

The tube has a life of at least 1000'flashes and gives reliable operation throughout this life.

3 shows an operating circuit for a twoelectrode tube which is similar in all its essentials to the three-electrode tube except .that it has no grid. Condensers II and each of four microfarads, are connected in parallel through one-megohm resistances 20, 20 and are charged through a two-megohm resistance l2 from the 7500 volt supply. A triggering impulse from the circuit III, which is essentially similar to that shown in Figure 2, is applied to the middle electrode of a three-electrode spark gap 2|, causing it to'break down. This puts the condensers ll and H in series across a spark gap 22 and'the tube I, causing the former to break down and an impulsive discharge to pass through the latter producing an intense flash-of light of extremely short duration. An alternative operating circuit using a three-electrode tube i in series with the two-electrode tube I is shown in Figure 4. The impulse from the triggering circuit 10, applied to the grid I through a 0.00001 microfarad condenser 23, causes the tube to break down and the tube l to discharge a two-microfarad condenser II. A ten-megohm resistance 24, is connected between the grid and cathode of the tube l other equivalent devices can replace the tube l in series with the two-electrode tube.

The two-electrode tube is mainly intended for applications where it is not, desirable to apply the high potential across the tube until the flash is desired, as for example in wet weather in the open. The tube and its 500,000 ohm shunt resistance 25 are the only components which need be exposed, and the leads connecting it to the remainder of the apparatus are at earth potential except when a flash occurs. Clearly, a threeelectrode tube can be used in this way by connecting'the grid and'rod electrode together, or by connecting the grid to the cylindrical electrode through a high resistance.

Figure 5 shows an operating circuit for a tube with two auxiliary anodes 26, 21. By the use of such auxiliary electrodes the discharge path can be extended to mm. They are connected to tapping points on a potentiometer comprising a 250,000-ohm resistance 28, a 500,000-ohm resist- IS, or to open the bias lead to the grid. or a (I ance 29, and a.70-megohm resistance 30. The

tube is triggered from the circuit i and discharges a four-microfarad condenser II.

The construction depicted in Figure 6 differs from that shown in Figure 1 only in that the cylindrical electrode I is replaced by a copper rod electrode 40 in alignment with the nickel rod electrode I and with the longitudinal axis of the tube. The electrode 40 is silver-soldered to a lead ll of tungsten sealed in the wall of the tube. The other components, numbered as in Figure 1, remain exactly as before and further description is unnecessary.

Figure 7 illustrates the preferred construction for light duty work and single flashes. In this case one of the main electrodes consists of a nickel rod 42 silver-soldered to a tungsten lead 43, and the other main electrode is in the form of a copper cup 44 mounted on a tungsten lead I so that it is co-axial with the electrode 42 and with the longitudinal axis of the tube. The third electrode I remains as previously described.

The electrodes, in the case of the two constructions last referred to, are activated with potassium in the manner described above, and the tubes are filled with argon or a mixture of argon and hydrogen, at, or approximately at, atmospheric ressure.

The construction depicted in Figure 8 is that which is preferred for heavy duty workwith multiple flashing. As the energy of discharge may be of the order of 50 joules per flash, and the peak current may amount to approximately 50,000 amperes, provision must be made for dissipating the heat developed in the main electrodes upon repeated flashing of the tube, especially when the rate of flashing is several hundreds per second. i

The envelope of this form of tube (Figure 8) is composed of the variety of heat-resistant glass known as W1." The main electrodes are relatively more massive than in the case of the constructions already described, this feature tending to obviate excessive heating of the components during operation of the tube. The anode consists of a thick-walled copper cup ll mounted on a copper stem 41 which is secured to a stout tungsten lead I held in a re-entrant seal 40 with a copper cap II.

The other main electrode takes the form of a stout copper rod i which is mounted on a tungsten lead 82 of the same diameter passing through a copper-capped seal as in the case of the lead 40. The triggering electrode consisls of a copper ring it supported by a tungsten lead II which is joined to a copper cap I fitted over the re-entrant seal.

The electrodes are activated with potassium hydride in the manner already described, and the tube is filled at 0 C. with argon and hydrogen at the respective pressures specified above. Should the anode it become too hot during operation of the tube, the potassium in its activating layer distils into the discharge path and a continuous arc discharge is liable to occur. Although this can generally be avoided by making the anode sufllciently massive to absorb and dissipate the heat generated, it may be found necessary to a118- ment the cooling of this electrode by fitting with cooling fins (not shown) I claim:

1. A cold cathode flash tube comprising a sealed tubular envelope containing a gaseous filling at a pressure within the range of 0.5 atmosphere to 3 atmospheres, said gaseous filling being composed solel of non-reactive gases, two rod- I posed solely of inert gases whereby the interior of the envelope is devoid of reactive gases, three electrodes mounted within said envelope with their axes of symmetry substantially co-incident with the axis of symmetry of said cylindrical envelope, the first of said electrodes being rodshaped and a second of said electrodes being ring-shaped and surrounding said first electrode, the surface layers of said electrodes being etched and crystalline and impregnated with electronemissive material of low work function, the interior of the tube and the electrodes being entirely free of excess electron-emissive material.

of 0.5 atmosphere to 3 atmospheres, three electrodes mounted within said tube, one of said electrodes being etched and crystalline and impregnated with electron-emissive material of low work function, and a second of said electrodes surrounding said first electrode, the interior of the tube and the electrodes being entirely free of excess electron-emissive material.

4. A cold cathode electron tube comprising a sealed cylindrical envelope of uniform cross-section containing a gaseous filling at a pressure within the range of 0.5 atmosphere to 3 atmospheres, said gaseous filling, being non-reactive and composed solely of inert gases. three electrodes mounted within said tube with their axes of symmetry substantially co-incident with the axis of symmetry of said cylindrical envelope, one of said electrodes being rod-shaped, the surface layer of said rod-shaped electrode being etched and crystalline and impregnated with electronemissive material, and a second of said electrodes being ring-shaped and surrounding said rodshaped electrode, the interior of the tube and the electrodes being entirely free of excess electron-emissive material.

5. A cold cathode electron flash tube comprising a sealed cylindrical envelope containing a gaseous filling at a pressure within the range of 0.5 atmosphere to 3 atmospheres, said gaseous fllling being composed solely of inert gases whereby the interior of the envelope is devoid of reactive gases, two rod-shaped electrodes mounted within said envelope with their axes co-incident with the axis of the tube and a ring-shaped triggering electrode surrounding one of said rod-shaped electrodes and having an axis substantially co-incident with the axis of the tube, the distance between said triggering electrode and the rodshaped electrode which it surrounds being less than the distance between said rod-shaped electrodes, the surface layers of said electrodes being etched and crystalline and impregnated with electron-emissive material having a low work function, the interior of the tube and the electrodes being entirely free of excess electron-emissive material.

a. A cold cathode ele'etron flash tube comprising a sealed tubular envelope containing a gaseous filling at a pressure within the range of 0.5 atmosphere to 3 atmospheres, a pair of rodshaped electrodes mounted within the envelope and having their axes co-incident with the longitudinal axis of the envelope, said rod-shaped electrodes being oppositely disposed and directed toward each other, a ring-shaped triggering electrode surrounding one of said rod-shaped electrodes, and a second ring-shaped triggering electrode between said rod-shaped electrodes and having an axis co-incident with the longitudinal axis of the envelope, the surface layers of said electrodes being etched and crystalline and impregnated with electron-emissive material, the interior of the tube and the electrodes being entirely free of excess electron-emissive material.

7. A method of manufacturing electron discharge tubes comprising the steps of mounting metallic electrodes symmetrically within a tubular envelope, introducing dilute acid into said envelope until the crystalline structure of the electrodes is shown, drying and evacuating said envelope to a hard vacuum, introducing potassium into said envelope, heating said envelope until the potassium penetrates the inter-crystalline surfaces of said electrodes, removing all excess and free potassium by evacuating and cooling said envelope, filling said envelope with an inert nonreactive gas at a pressure within the range of 0.5 to 3 atmospheres, and sealing said envelope.

8. A method of manufacturing an electron discharge tube comprising the steps of mounting metallic electrodes symmetrically within a tubular envelope, introducing dilute acid into said envelope until the crystalline structure of the electrodes is shown, drying and evacuating said envelope to a hard vacuum, distilling an electronemissive element into said envelope, heating said envelope until said element penetrates the intercrystalline surfaces of said electrodes, evacuating all of the free portion of said element from the envelope, introducing hydrogen at a pressure of a few millimeters of mercury into said envelope while passing a glow discharge of low current density between the electrodes until layers of the hydride of said element are formed on the electrodes, filling said envelope with an inert nonreactive gas at a pressure within the range of 0.5 atmosphere to 3 atmospheres, and sealing said envelope.

9. A method of manufacturing a metal electrode comprising the steps of etching said electrode with a dilute acid until the inter-crystalline surfaces of said electrode are shown, coating 10 and impregnating said electrode with a material having a low work function 'and which penerales the intercrystalline surfaces of said electrode, and removing from the surface of said electrode all of said material which is excess and free.

10. A method of manufacturing a metal electrode comprising the steps of etching said electrode, coating and impregnating said electrode with electron-emissive material, and removing all excess and free electron-emissive material from the surface of said electrode.

11. An electrode comprising an etchedmetallic crystalline member, a coating of electron-emissive material on the surface of said member, the inter-crystalline surfaces of said member being impregnated with said electron-emissive material so as to provide a reserve supply of said material for said coating.

12. A cold cathode electron tube comprising a sealed tubular envelope containing a non-reactive gaseous filling at a pressure within the range of 0.5 atmosphere to 3 atmospheres, three electrodes mounted within said tube, the first of said electrodes being rod-shaped, said first electrode being etched and crystalline and impregnated with electron-emissive material of low work function, and a second of said electrodes surrounding said first electrode, said second'electrode being ring-shaped and spaced substantially from said envelope, said first electrode protruding beyond said second electrode toward said third electrode, the interior of the tubes and the electrodes being entirely free of excess electron-emissive material.

JOHN WESLEY MITCHELL.

REFERENCES CITED The following references are of record in the file of this patent:

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